docs: def get_css_classes(cls, instance):
          """
          Returns a list of CSS classes to be added as class="..." to the current HTML tag.
          """
          css_classes = []
          if hasattr(cls, 'default_css_class'):
              css_classes.append(cls.default_css_class)
          for attr in getattr(cls, 'default_css_attributes', []):
              css_class = instance.glossary.get(attr)
              if isinstance(css_class, six.string_types):
                  css_classes.append(css_class)
              elif isinstance(css_class, list):
                  css_classes.extend(css_class)
          return css_classes